A four-stage sequence (left to right) showing the possible extreme temperature evolution for WISE J0304-2705. When young, the object was as hot as a star, shining with a temperature of at least 5,100° F (2,800° C) for about 20 million years. After 100 million years or so, it had cooled to 2,700° F (1,500° C), and by a billion years its temperature was around 1,800° F (1,000° C). The final stage is billions of years later, when WISE J0304-2705 had cooled to its current planetary temperature of 210°–300° F (100°–150° C).

John Pinfield, 2014

Astronomers have discovered an extremely cool object that could have a particularly diverse history; although it is now as cool as a planet, it may have spent much of its youth as hot as a star.

The current temperature of the object is 210°–300° Fahrenheit (100°–150° Celsius), intermediate between that of Earth and Venus. But the object shows evidence of a possible ancient origin, implying that a large change in temperature has taken place. In the past, this object would have been as hot as a star for many millions of years.

Called WISE J0304-2705, the object is a member of the recently established “Y dwarf” class, the coolest stellar temperature class yet defined, added to the end of the sequence OBAFGKMLT — for historical reasons this is not in alphabetical order but follows a decline in temperature from O to T. Although its temperature is not far off that of our own world, the object is not like the rocky Earth-like planets and instead is a giant ball of gas like Jupiter.

The international discovery team, led by David Pinfield from the University of Hertfordshire in the United Kingdom, identified the Y dwarf using the Wide-field Infrared Survey Explorer (WISE) observatory — a NASA space telescope that since its launch in 2009 has imaged the entire sky in mid-infrared light — rather redder than the reddest light we can see with our eyes. The team also dispersed the light emitted by the Y dwarf into a spectrum, which allowed them to determine its current temperature and better understand its history.

Only 20 other Y dwarfs have been discovered to-date, and amongst these, WISE J0304-2705 is defined as “peculiar” due to unusual features in its emitted light spectrum. “Our measurements suggest that this Y dwarf may have a composition and/or age characteristic of one of the galaxy’s older members” said Pinfield. “This would mean its temperature evolution could have been rather extreme. Despite starting out at thousands of degrees, this exotic object is now barely hot enough to boil a cup of tea.”

The reason that WISE J0304-2705 underwent such extensive evolutionary cooling is because it is “sub-stellar” — its interior never became hot enough for hydrogen fusion, the process that has kept the Sun hot for billions of years. And without an energy source maintaining a stable temperature, cooling and fading was inevitable.

If WISE J0304-2705 is an ancient object, then its temperature evolution would have followed the stages shown in the illustration. During the first 20 million years or so of its life, it would have had a temperature of at least 5100° F (2800° C), the same as red dwarf stars like Proxima Centauri — the nearest star to the Sun. After 100 million years, it would have cooled to about 2700° F (1500° C) with silicate clouds condensing out in its atmosphere. At a billion years of age, it would have cooled to about 1800° F (1000° C), cool enough for methane gas and water vapor to dominate its appearance. And since then, it has continued to cool to its current temperature of 210°–300° F (100°–150° C).

WISE J0304-2705 is as massive as 20 to 30 Jupiters combined, so somewhere between the least massive stars and typical planets. But in terms of temperature, it may have actually “taken the journey” from star-like to planet-like conditions.

Having identified WISE 0304-2705, Pinfield’s team made crucial ground-based observations with some of the world’s largest telescopes — the 8-meter Gemini South Telescope, the 6.5-meter Magellan Telescope, and the European Southern Observatory’s 3.6-meter New Technology Telescope, all located in the Chilean Andes.

“The ground-based measurements were very challenging, even with the largest telescopes,” said Mariusz Gromadski. “It was exciting when the results showed just how cool this object was and that it was unusual.”

“The discovery of WISE J0304-2705 with its peculiar light spectrum poses ongoing challenges for the most powerful modern telescopes that are being used for its detailed study,” said Maria Teresa Ruiz from the Universidad de Chile.

WISE J0304-2705 is located in the constellation of Fornax the Furnace in the southern hemisphere of the sky and is between 33 and 55 light-years away.

There is currently no lower limit for Y dwarf temperatures, and there could be many even cooler and more diverse objects undetected in the solar neighborhood. WISE went into hibernation in February 2011 after carrying out its main survey mission. However, by popular demand, it was revived in December 2013 and is continuing to observe as part of a 3-year mission extension.

“WISE gives us wonderful sensitivity to the coolest objects.” said Pinfield, “And with 3 more years of observations, we will be able to search the sky for more Y dwarfs and more diverse Y dwarfs.”